Re: [RFC PATCH v2 04/23] sched/topology: Introduce a method to calculate the llc distance

[Peter Zijlstra]

On Thu, Aug 27, 2026 at 08:27:57PM +0800, Jianyong Wu wrote:
> The previous patch addresses NUMA inter-node distance handling. This
> patch shifts focus to the LLC layer and introduces a scheme to compute
> pairwise distances between sibling LLCs inside one NUMA node.
> The calculation only guarantees distinct distance values for all peers
> when viewed from a single reference LLC; it does not enforce globally
> unique distances for every arbitrary LLC pair on the node.
> 
> One valid calculation scheme is defined by the formula below:
> 
>   dist(LLC1, LLC2) = (rank1 + rank2) % k + 1
> 
> Constraints: LLC1 and LLC2 must reside on the identical NUMA node.
> rank1 = local index of LLC1 within its node; rank2 follows the same rule.
> k = total count of LLCs available on the node.
> 
> The formula never yields zero, so diagonal self-distance entries are
> explicitly hardcoded to 0.
> 
> For demonstration, consider a NUMA node with four LLCs (local ranks 0~3):
>         LLC0    LLC1    LLC2    LLC3
> LLC0    0       2       3       4
> LLC1    2       0       4       1
> LLC2    3       4       0       2
> LLC3    4       1       2       0
> 
> Sorting each matrix row in ascending order generates a unique LLC affinity
> sequence, which drives thread migration policies implemented in follow-up
> patches.

Aah, now I see. I got confused by your sched_cache_node_distance naming,
and thought it was cache-to-cache distance.